If buy a nintendo switch 2, then buy the switch 2 upgrade for my games, will I have 2 different save file on the switch and switch 2, or will it be one shared one ?

So, I'm asking that mainly because I've been wanting to get a new island in ACNH for a while now, but I also don't want to erase my existing island

If I buy the switch 2 upgrade for my copy of ACNH, will I be able to have a different island on each console or will it be a shared one on both consoles ?